TEHRAN (Reuters) -- Iran has renewed a travel ban on an Iranian-American student who was detained last year on security-related charges but later freed on bail, the judiciary has said.

Esha Momeni was detained in mid-October during a visit to Iran from the United States to see family and carry out research on the women's movement. She was freed on November 10 after spending almost four weeks in a Tehran prison.

Judiciary spokesman Alireza Jamshidi, who late last year said he saw no obstacle to Momeni leaving Iran, said she had been banned again from departing because of a new charge against her that was being investigated. He did not give details.

"About two weeks ago the ban was reimposed," he told a news conference.

"I was told that she was given back her passport but...a new [charge] has been raised and it is being investigated and that is why the ban was reimposed," he said. "I think the ban will last for about a month or so before it is withdrawn."

There was no immediate comment from Momeni or her family.

The judiciary has said Momeni was accused of acting against national security and of making propaganda against the Islamic republic's system of government, a common charge against Iranian dissidents. Momeni holds dual Iranian and U.S. citizenship.

Women's rights activists say Momeni was detained after interviewing campaigners for a film she was working on as part of her studies in California. She was released after a deed to her family's home in Iran was posted as bail.

Activists say dozens of them have been detained since they launched a campaign in 2006 to collect 1 million signatures in support of demands for changes in laws they say deny women in Iran equal rights in matters such as divorce and child custody.

Most were freed after a few days or weeks. Iran denies it discriminates against women.

Iran detained four Iranians with dual U.S. citizenship on security charges in 2007, drawing protests from Washington.

They were later freed on bail after state television showed two of them confessing to acting against Iranian national security and spying.